The present invention relates to a grit selector or trap for selectively catching grit of all sizes desired and passing all organics into a waste treatment plant.
Grit is one of the most unpredictable and difficult materials a sewage treatment plant must handle. Grit can be defined as the heavy mineral matter present in sewage. It is principally made up of sand and soil, but frequently contains cinders, coffee grounds, seeds, corn, and other coarse sediment which reaches a sewage disposal plant. It is desirous to remove this material as it cannot be treated, reduced in size, or eliminated by treatment methods. It presents a problem to waste treatment as it is hard and abrasive. It wears pumps and other mechanical devices. It is heavy and accumulates in clarifiers, treatment basins, digesters, etc., where it must often be removed by hand.
This problem has been apparent for years. In conventional grit removal devices, control of sewage velocity is utilized to remove the grit. The basins are generally long and narrow and have some type of velocity controlling outlet. The problem of selective removal of grit particles is complicated by fluctuations in rate of flow, especially those that accompany storm rainfalls. Aside from subdividing the grit chamber into several compartments that can be taken in and out of service, a solution to the velocity problem is sought by combining opportunity for sedimentation of wanted particles with scour or resuspension of unwanted particles. This requires the provision of adequate surface area and maintenance of an adequate displacement velocity. Fluctuations in flow require, ideally, that both (1) a constant value of Q/A and (2) a constant displacement velocity V be maintained in the chamber. Ordinarily a compromise solution is offered in order to keep the required structure simple. The cross-section of the chamber at right angles to the direction of flow is made uniform throughout its length, and its shape is so chosen that the displacement velocity is held substantially constant at all depths of flow. For this purpose, a flow control device, such as a proportioned flow weir, a vertical throat, or a standing wave flume, may be placed at the end of the chamber. The velocity at maximum flow is then made large enough to ensure deposition of wanted large and heavy particles, and selective movement and resuspension of smaller and lighter particles which will settle as flow is reduced, being cared for by the constant scouring action of the flowing water.
Once the grit has settled in the chamber, some type of removal procedure must be provided. Many of the older plants employed hand or manually cleaned grit chambers. Most plants of more recent design provide some type of continuous mechanical collection of the grit. Chain and scraper, bucket conveyors, and circular scraper mechanisms have been utilized.
There are many significant disadvantages of the types of conventional grit chambers discussed above. It is difficult to control the velocity through the grit chamber and this results in deposition of unwanted organic matter and possibly a wash through of some grit. This means many organics are settled out that could be treated by various means. It also means the grit cannot be disposed of readily because of its pollution load. Conventional grit chambers further have grit removal systems which include collection mechanisms (i.e., chains, buckets, bearings) below the water surface in a high abrasion atmosphere. Another drawback of conventional grit chambers is their large size.